[llvm] [IR] Use SmallDenseSet (NFC) (PR #79556)

[Kazu Hirata via llvm-commits]

https://github.com/kazutakahirata created https://github.com/llvm/llvm-project/pull/79556

The use of SmallDenseSet saves 0.39% of heap allocations during the
compilation of a large preprocessed file, namely X86ISelLowering.cpp,
for the X86 target.  During the experiment, WL.size() was 2 or less
99.9% of the time.  The inline size of 4 should accommodate up to 2
entries at the 3/4 occupancy rate.


>From 817096eff6d0d0d480ce95d23029ddd947577c58 Mon Sep 17 00:00:00 2001
From: Kazu Hirata <kazu at google.com>
Date: Wed, 24 Jan 2024 09:51:51 -0800
Subject: [PATCH] [IR] Use SmallDenseSet (NFC)

The use of SmallDenseSet saves 0.39% of heap allocations during the
compilation of a large preprocessed file, namely X86ISelLowering.cpp,
for the X86 target.  During the experiment, WL.size() was 2 or less
99.9% of the time.  The inline size of 4 should accommodate up to 2
entries at the 3/4 occupancy rate.
---
 llvm/lib/IR/Instruction.cpp | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/llvm/lib/IR/Instruction.cpp b/llvm/lib/IR/Instruction.cpp
index f9a38e48166c9ab..904ce17fb0e7c97 100644
--- a/llvm/lib/IR/Instruction.cpp
+++ b/llvm/lib/IR/Instruction.cpp
@@ -1167,9 +1167,7 @@ void Instruction::copyMetadata(const Instruction &SrcInst,
   if (!SrcInst.hasMetadata())
     return;
 
-  DenseSet<unsigned> WLS;
-  for (unsigned M : WL)
-    WLS.insert(M);
+  SmallDenseSet<unsigned, 4> WLS(WL.begin(), WL.end());
 
   // Otherwise, enumerate and copy over metadata from the old instruction to the
   // new one.